Human papillomavirus (HPV) infection was reported as a cause of cervical cancer in the 1980's. The relationship between cancer malignancy and HPV genotype was considered to be of interest. More than 120 HPV types have since then been identified. Thirteen of the HPV types are recognised as being oncogenic in mucosal epithelia. These types include HPV 16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59 and 66.
Human papillomavirus infections are confined to the epithelium, which limits their contact with the immune apparatus and they are poorly immunogenic. Thus serology is not a useful test for HPV. Similarly, culture is not a useful diagnostic technique. Although it is possible to reproduce the virus life cycle in vitro, this is a highly sophisticated tissue culture technique requiring in vitro differentiating epithelial cell cultures. Furthermore, high grade cervical neoplasias and cancers do not produce virus. Therefore, HPV testing relies on the detection of viral nucleic acids.
HPV testing has been shown to have a higher sensitivity than the PAP test (Papanicolaou test) for high-grade cervical dysplasia, although its specificity is lower due to the high prevalence of benign, transient HPV infections.
Many countries have adopted HPV testing as a secondary screening (triage) test for low-grade and equivocal dysplasia and its use as in primary screening has also been suggested.
The dominant HPV test is at present the Digene HCII test, whose utility is supported by a large body of scientific literature. In this test HPV DNA in the sample is hybridised to complementary RNA and the resulting DNA-RNA hybrids are captured by antibodies on a solid surface where they subsequently bind antibody-enzyme conjugates that are detected by enzyme-catalysed chemiluminescence. The LCII test does not distinguish between the different oncogenic HPV types.
It is also possible to detect HPV in fixed cells using in-situ hybridisation with probes labelled either with fluorescent moieties (fluorescence in-situ hybridisation, FISH) or with haptens that can bind to an enzyme conjugate which generates an insoluble coloured precipitate in situ. In situ hybridisation can provide valuable information on the location and physical state of HPV, but type specific detection of multiples of HPV types in the same sample is impracticable.
Many screening authorities express a preference for genotyping tests in secondary screening since this allows serial transient infections which are benign, to be distinguished from type-specific persistent infections which are not benign. In addition, some HPV types, particularly HPV16, are markedly more oncovirulent.
Finally, the use of a genotyping test provides useful epidemiological information which will be of particular value in monitoring the population effects of vaccination against HPV16 and HPV18.
The available HPV genotyping tests are mostly based on PCR followed by reverse hybridisation in some format. The PCR step targets the L1 or the E1 genes. PCR amplicons are labelled under amplification and hybridised to immobilised probes on a microarray, a membrane strip or in the wells of a microtiter plate. The bound labelled amplicon is then detected, either directly by fluorimetry if the amplicon is labelled with a fluorescent moiety or using enzyme conjugates that bind to the label and which are detected using colorimetric or luminometric techniques. Such tests involve handling of PCR amplicons in the open laboratory which necessitates stringent hygienic precautions. This, combined with the high price of such tests places the available tests beyond the reach of most mass-screening programs.
A genotyping test that detects mRNA from the viral oncogenes E6 and E7 using SDA (sequence dependent amplification) is also available. However, this method detects only five of the thirteen oncogenic types and only when there is active oncogene expression.
WO 2007115582 A1 describes sets of PCR primers and probes for detection of multiple HPV types by real-time PCR. These sets involves a complex combination of primers. In the method according to present invention, only one forward primer and four reverse primers are required. This simplifies quality control and manufacture and reduces cost. Further, the genes that are targeted in WO 2007115582 A1 are subject to intratypic sequence variation. This variation is not taken into account. Thus false negative results may occur in clinical samples that contain HPV sequence variants other than the prototype sequences. According to the present invention known sequence variation within the target region of the L1 gene has been taken into account prior to primer and probe design and all variant positions are either avoided or accounted for by modification of primer sequences.
Further, in WO 2007115582 A1 it is stated that sensitivities down to 100 copies for all targeted HPV types are achieved. However, the experiments described do not include the use of carrier DNA. This gives an unrealistically optimistic estimate of sensitivity as the human DNA present in clinical samples suppresses PCR sensitivity. The sensitivity of the method according to the present invention is measured in the presence of 500 ng of human DNA, which correspond to 7.5×104 human cells and thus accurately mimics the conditions in a cell-rich clinical sample. Thus the present invention provides enhanced sensitivity under realistic conditions.
Further, WO 2007115582 A1 describes the use of a single tube 13-plex multiplex PCR test. However, according to the state of the art there does not seem to be an instrument capable of performing such a test. The method according to the present invention may be operated on existing obtainable instruments in the format proposed or a variant or an analog thereof.
In WO 2009011472 A1 the target gene which has been used for the PCR tests is E1. However, this gene is a hotspot for deletions and insertions during cancer development. The probability of a false negative result increases thus with the severity of disease. The method according to the present invention targets the L1 gene, which is much less prone to deletion.
Further, in WO 2009011472 A1 the methods for detection and typing of HPV involve handling of PCR products in the open laboratory. This involves the risk of carry-over contamination. The method according to the present invention is performed in a closed-system which eliminates this risk, thus ensuring more reliable results and eliminating the extra labour required for decontamination of equipment and facilities after open-laboratory post-PCR work.
Based on the above, there is thus a need in the field for new methods, primers, probes and kits for detection and typing of HPV which are highly sensitive, reproducible and less expensive than the existing tests and which thus are appropriate for mass-screening programs.